Thin film transistor-liquid crystal display (TFT-LCD) is known as the display required for the high pixel density and quality. In general, the TFT-LCD includes a bottom plate formed with thin film transistors and pixel electrodes and a top plate formed with color filters. The liquid crystal is filled between the top plate and the bottom plate. In each unit pixel, a capacitor and a further capacitor are provided which are formed by virtue of the TFT serving as the switching element of the unit pixel. In the operation, a gate signal voltage is applied to the TFT that is the switching element of each unit pixel. The TFT receives the gate signal voltage, it is turn on so that data voltage carrying image information can be applied to the corresponding pixel electrode and the liquid crystal via the TFT. When the data voltage is applied to the TFT, the arrangement of the liquid crystal moleculers is change, thereby changing the optical properties and displaying the image. Supplying of voltage to each pixel electrode is carried out via a single line by a corresponding thin film transistor which is switched by a gate voltage supplied via a gate bus line.
In U.S. Pat. No. 5,166,086 proposed by Takeda et al., he disclosed a method to form a TFT-LCD. In the patent, Takeda provides a TFT array with reduced parastic capacitances. Further, Possin et al. disclosed a method to form a TFT-LCD by using back-side photoresist exposure. Please refer to U.S. Pat. No. 5,010,027. Possin teaches a method for forming a self-aligned thin-film transistor which controls the overlap width of the gate electrode with each source and drain electrode to an optimum distance.
Typically, the channel region of the self-aligned TFT-LCD is defined by the gate structure as a mask. The lightly doped drain (LDD) region or the offset structure is formed by a further photomask. The object of the present invention is to provide a method to form the offset structure without a further photomask as aforesaid.